1. Field of the Invention
The present invention relates to a water base ink set for ink-jet recording including a black ink, and an ink-jet recording apparatus which accommodates the same.
2. Description of the Related Art
The ink discharge system, which has been hitherto known as the ink-jet recording method, includes, for example, the electrostatic attraction method, a method in which mechanical vibration or displacement is applied to the ink with a piezoelectric element or the like, and a method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. Ink droplets are formed by means of the ink discharge method as described above, and all or a part of the ink droplets are adhered to a recording objective such as paper to perform the recording. Those known and used as the ink to be employed for the ink-jet recording method as described above include those in which a variety of water-soluble dyes or water-dispersible pigments are dissolved or dispersed in liquid media each comprising water or a combination of water and water-soluble organic solvent.
In order to adequately perform the recording for a long period of time by using the ink as described above, for example, it is necessary to satisfy the following conditions. That is, the characteristic values (for example, those of the viscosity, the surface tension, and the density) of the ink are appropriate values. No deposited matter is generated and no physical property value is changed by the heat or the like (to avoid any clog-up at the nozzle or the orifice of the recording apparatus and discharge the ink stably). The recorded image is excellent, for example, in water resistance and light resistance.
When the recording is performed with an ink-jet printer by using an ordinary water base ink, exclusive ink-jet paper is sometimes used, in order to obtain good printing qualities without involving any blurring of the ink. However, in recent years, it is more demanded to perform the recording on the regular paper rather than the recording on the exclusive ink-jet paper, in view of the running cost and the consideration of the environment.
Further, as for the color of the ink, the color ink-jet printer is overwhelmingly demanded as compared with the monochrome ink-jet printer in the market directed to home use and office use.
However, when the recording is performed on the regular paper, a problem arises such that the printing quality is remarkably deteriorated as compared with the case in which the recording is performed on the exclusive ink-jet paper. For example, the following problem arises. That is, when the ink permeates into the recording paper, then the ink spreads non-uniformly, and the edge of an image portion is notched, making it impossible to obtain any sharp edge of the image portion. This phenomenon is generally called “feathering”.
Further, in the case of the color ink-jet printer, another problem arises in addition to the problem described above such that the inks are mixed with each other at the portion (hereinafter referred to as “boundary”) at which the different colors are disposed adjacently, and the both inks are blurred, resulting in the deterioration of the printing quality. This phenomenon is generally called “color bleed”. The color bleed tends to be conspicuous, for example, especially when letters or lines are recorded with the black ink having a relatively dark color on the background of the color ink, i.e., yellow, magenta, cyan or the like having a relatively bright color.
Many techniques have been used in order to solve the problems as described above, suppress the feathering and the color bleed, and obtain the good printing quality.
At first, in order to reduce the feathering, a method is generally used, in which the surface tension of the ink is increased. The higher the surface tension is, the better the successfully obtained printing quality is, in which the feathering scarcely appears.
A technique, which is used to reduce the color bleed, is a method in which the surface tensions of the color ink and the black ink are lowered so that they are in equivalent degrees (within ±10% of the black ink). In most cases, the surface tension is adjusted to be about 28 to 38 mN/m.
In the case of the ink set having been hitherto used, no special consideration is made for the viscosity of the ink. For example, in view of the discharge stability, the viscosities of the black ink and the color ink are adjusted to be in approximately equivalent degrees.
Unlike the general techniques as described above, it has been suggested in Japanese Patent Application Laid-open No. 6-106841 that a pigment is used for a coloring agent of a black ink, and a precipitating agent is added to a color ink in order to precipitate the black pigment. This suggestion is based on the following method. That is, the black pigment is aggregated and precipitated by using a polyvalent metal salt as the precipitating agent dissolved in the color ink, at the boundary at which the black ink and the color ink make contact with each other. Accordingly, the fluidity of the black pigment in the black ink is suppressed to avoid the color bleed.
As described above, in order to reduce the feathering and the color bleed, the techniques corresponding to the respective cases have been hitherto used.
However, most of the conventional inks as described above have involved the following problem. That is, when the recording is performed on the regular paper, any one or all of the feathering and the color bleed of the black ink and the color ink are caused. It has been impossible to obtain any satisfactory printing quality in which all of them are simultaneously improved.
As described above, there has been the following problem. That is, if the surface tension is about 28 to 38 mN/m as in the conventional ink set, and the viscosity is approximately equivalent to one another in ratio, then the feathering has been unsatisfactory. If it is intended to increase only the surface tension, it has been impossible to satisfy the color bleed. It has been impossible to simultaneously satisfy the requirements for the color bleed and the feathering.